Although screw-type extruders useful for extruding polymeric materials are well known, problems are encountered in trying to extrude composite materials comprising significant amounts of wood fibers in combination with a polymeric resin in conventional extruders. Such problems include, for example, high shear rates which can increase temperatures and cause fiber breakage. High temperatures can reduce polymer viscosity below the level needed to produce a uniform product with good fiber dispersion, and can cause charring of the wood fiber. Fiber breakage can adversely affect fiber alignment and the physical properties of the extrudate.
An extruder is therefore needed that is specially adapted for extruding such composite materials.